Add best-of-3 retry for flaky API, cache past year data

- Series searches now retry 3 times and merge results by ASIN
- Past years are cached in static/data/ and committed to repo
- Only current year + future are re-fetched on each build
- Significantly faster builds for the weekly cron

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>